In either case, Boris has played a Billy Bunter peevish trick to his local audience.
The letter was addressed to Tusk who would send it back with “you forgot to sign it.” and then Boris would be compelled under the Benn Act to sign it.
Boris may be delaying to get his deal signed by parliament.
Either way, the leaders who are from the countries who would be most harmed - Ireland and France are the ones who will push the move. Ireland has been setting EU policy on Brexit since 2017 - it’s key concern is the frictionless border. France wants the UK out. Merkel is slowly getting to Macron’s position, but still the EU position will be decided by Varadkar
maybe a signature would require a separate court decision or even another act of parliament, presuming tusk bothers to inquire at all... all in the timing...
Had to look this one up. My knowledge of Irish government is limited. Explain why. https://en.m.wikipedia.org/wiki/Taoiseach
Is the key vote?